The present invention relates to a foamed article having a solid skin layer and a core part both made of single thermoplastic resin composition and to a process for producing such foamed article.
Thermoplastic resins, in particular, polyolefin resins have found wide applications in industry. Since polyolefin resins has no content of chlorine, they do not emit noxious gases on incineration. The extent of application thereof has rapidly been developed as environment-favoring materials, since they permit also recycled use. In particular, in the field of automotive industry, there have been a demand for parts made of a polyolefin resin which are light-weighing and are superior in the stiffness and so on.
Injection molded foamed articles, which are produced by injection-molding such a polyolefin resin while causing it simultaneously to foam up, have been in practical use as the polyolefin resin products. In general, a foamed article produced by injection-molding a polyolefin resin composition containing a foaming agent with simultaneous foaming is inferior in the appearance and in the strength and has lower commercial value as compared with molded article without foaming, since foam cells are found also within the surface layer of the foamed article.
Therefore, it has been practised, in an application field, for example, automotive parts, in which greater importance is placed on seeking for better appearance, to provide the foamed article with a skin layer, by covering the inner face of the cavity of foaming mold with a skin layer prepared preliminarily in a separate process step and, then, injecting the molten foaming resin composition thereinto, in order to build up a foamed article covered with the skin layer, or by applying a skin layer onto the injection molded foamed article.
However, the conventional practice given above has problems that the procedures are complicate and that the spent or wasted articles are difficultly brought into recycled use, as the skin layer and the core part of the foamed product are made of different resins.
For solving these problems, a technique has been proposed in which a solid skin layer is formed on the external face of the foamed molded article at the same time with the molding of the foamed article.
For example, Japanease Patent Kokai Hei 4-144721 A discloses a technique in which a molten resin is injected into a metal mold while receding the mold. This prior technique has some problems that the molten resin can hardly be filled fully up to peripheral portions of the mold cavity with a trend of easy occurrence of short shot, therefore thicker skin layer is difficult to obtain and the distribution of sizes of foam cells became rough.
Japanese Patent Kokai Hei 4-214311 A discloses a technique in which a molten resin is injected into a metal mold while receding it and, then, the metal mold is moved once to the direction for closing it to form a solid skin layer, whereupon foaming is effected under recession of the metal mold. However, this prior technique has problems in that wavy flow marks (in the following referred to sometimes as swirl marks) may be apt to occur on the surface of the foamed article and that the process steps are intricate and require extended process cycles. When flow marks, such as the swirl marks, are formed on the surface of the foamed article, the appearance of the article is debased, prohibiting thus use of such an article for a non-coated article, such as those recently found uses for the interior and exterior furnishings of automobile.
Japanese Patent Kokai Hei 8-300391 A describes a technique in which a molten resin is injected into a metal mold held in a fixed state and the mold is then cooled in the state as such, whereupon foaming is effected while receding the metal mold. However, this prior technique has problems in that the foam cells formed are irregular in sizes resulting in an inferior heat insulation and lower mechanical strength, that the process requires prolonged procedures and that swirl marks due to flow of the molten resin on the surface of the foamed article are not able to exclude.
Japanese Patent Kokai Hei 10-128795 A described a technique in which a molten resin is injected into a metal mold in a fixed state of the mold to fill up it, whereupon foaming is effected at once while receding the metal mold. However, this prior technique has some problems in that some reduction in the foaming expansion ratio should be endured, when it is attempted to increase the solid skin layer thickness, and vice versa.
Starting from the stand of the technique as given above, the inventors reached a solution in which a part of the raw material is first injected into a restricted cavity of the metal mold, whereupon the remaining part of the raw material is injected into the cavity while relieving the restriction of cavity space. By this technical measure, it has now been made possible to produce a molded foamed article having a thicker solid skin layer with a high foaming expansion ratio.
The present invention further provides a technique in which occurrence of flow marks, such as swirl mark and so on, can be excluded, whereby foamed articles capable of serving for non-coated articles as, for example, automobile interior and exterior parts can be produced.
The first object of the present invention is to provide a process for producing a light-weight foamed article of superior stiffness from a thermoplastic resin composition in an easy and efficient manner, wherein the said foamed article has a solid skin layer made of the same thermoplastic resin composition as that of its core part and the sizes of the foam cells are relatively uniform with better cell configuration.
The second object of the present invention is to provide a process in which a foamed article made of a thermoplastic resin composition and having a solid skin layer of relatively greater thickness can be produced at a higher foaming expansion ratio in an easy and efficient manner.
The third object of the present invention is to provide a process which can afford to produce a foamed article made of a thermoplastic resin composition in an easy and efficient manner, in which a solid skin layer having no flow mark, such as swirl mark, and, thus, providing better appearance can be formed easily.
The above objects are attained by the present invention which consists in a process for producing a foamed article of a thermoplastic resin composition and in a foamed article produced by such process as given in the following:
(1) A process for producing a foamed article of a thermoplastic resin composition by causing the said resin composition to foam up in a cavity of a mold, comprising
a primary injection step in which a part amount of the thermoplastic resin composition is injected into the mold cavity defined by a stationary mold element and a movable mold element settled at a position confining the mold cavity to a smaller volume than the total volume of the thermoplastic resin composition necessary for making up the complete foamed article, while imposing a clamping pressure on the movable mold element so as to settle it at the said position,
a secondary injection step subsequent to the primary injection step, in which the residual amount of the thermoplastic resin composition is further injected into the mold cavity, while drawing the movable mold element back from the position settled for the primary injection step so as to increase the cavity volume, and
a foaming step subsequent to the secondary injection step, in which injection of the thermoplastic resin composition is stopped and the movable mold element is further drawn back so as to permit the thermoplastic resin composition to foam up,
wherein the thickness L0 of the mold cavity at the start of the primary injection step is in the range from 1.0 to 1.5 mm, the injection time of the primary injection step is 1.5 seconds or less and the pressure imposed on the movable mold element in the primary injection step is in the range from 5 to 20 MPa with respect to the sectional area of the mold cavity in the projection onto the mold base plane.
(2) The process as defined in the above (1), wherein the ratio of the thickness L0 of the mold cavity at the start of the primary injection step relative to the thickness L1 at the end of the secondary injection step, namely, L0/L1, is in the range from 0.3 to 1.0.
(3) The process as defined in the above (1) or (2), wherein the recession of the movable mold element subsequent to the secondary injection step begins within 5 seconds from the termination of the secondary injection step.
(4) The process as defined in any one of the above (1) to (3), which comprises further a step of compressing the foamed article, in which the movable mold element is pressed onto the foamed article within 60 seconds from the termination of the foaming step.
(5) The process as defined in any one of the above (1) to (4), wherein the thermoplastic resin composition comprises a polyolefin resin and a foaming agent.
(6) The process as defined in the above (5), wherein the polyolefin resin is a polypropylene resin.
(7) The process as defined in the above (5) or (6), wherein the polyolefin resin has a melt flow rate, determined according to ASTM D 1238 at 230xc2x0 C. under a load of 2.16 kg, in the range from 30 to 200 g/10 min.
(8) A foamed article of a thermoplastic resin composition, which is produced by the process as defined in any one of the above (1) to (7).
(9) The foamed article as defined in the above (8), wherein the solid skin layer has a thickness in the range from 0.1 to 0.7 mm.
(10) The foamed article as defined in the above (8) or (9), wherein the foaming expansion ratio is in the range from 1.05 to 5 times of the non-expanded original volume.
(11) Automotive parts as foamed articles, which are produced by the process as defined in any one of the above (1) to (7).